WO2002035205A2

WO2002035205A2 - Process for detecting or quantifying a biological reaction using superparamagnetic label

Info

Publication number: WO2002035205A2
Application number: PCT/US2001/032598
Authority: WO
Inventors: Qi Chen
Original assignee: Binax, Inc.
Priority date: 2000-10-20
Filing date: 2001-10-22
Publication date: 2002-05-02
Also published as: WO2002035205A9; US20030124745A1; WO2002035205A3; WO2002035205A8; JP2004530103A

Abstract

A process is described in which superparamagnetic particles are first conjugated or adsorbed to a group of identical biomolecules,such as a group of antibodies, and the conjugates or adsorbates are then reacted with a group of biological binding partner molecules to form a tightly bound, three-dimensional mass of interlinked biomolecules and bound superparamagnetic particles. The mass is exposed to a magnetic field for the shortest period need to induce magnetization of the superparamagnetic particles and the field is then inmediately removed. The superparamagnetic particles in the mass exhibit in concert a measurable nonpermanent aggregative magnetization for a period of at least twenty minutes which can be used to quantitate the amount of biological binding partner present by comparison with preestablished standards or to confirm the presence of a biological binding partner in a test sample.

Description

PATENT APPLICATION

This application is a continuation-in-part of pending U.S. application No. 09/692,463 filed October 20, 2000.

The present invention relates to a new detection system for recognizing and/or

monitoring biological, including biochemical, reactions and especially -immunochemical

reactions. In this system, the introduction of superparamagnetic force effectively facilitates

accurate detection of the degree of such a reaction based upon the concentration, or

number, of molecules of one reactant that have participated in the reaction. The invention

will find ready application in a variety of in vitro uses, including immunoassays,

chromatographic molecular separations, nucleic acid probe analyses, pesticide residue analyses, oligonucleotide probes, and other areas in which biological, including

biochemical, reactions are now observed or measured or where such observations and

measurements plainly could usefully be made even though not yet reported.

BACKGROUND OF THE INVENTION

Various references describe the use of certain colloidal metallic particles, and specifically of colloidal gold, as identifiers for unusual cells or molecular entities that may

be present in biological tissues and as tags for biological, including biochemical, molecules

in a variety of immunoassays, nucleic acid probe analyses, chromatographic separations,

oligonucleotide probes and myriads of other specific applications where the monitoring of a

biological reaction, the identification of one or more disease-causing organisms, the

identification of specific moieties that participate in a particular biological reaction or that

may

disrupt the normal functioning of such a reaction, the identification of moieties that trigger pathological reactions, and similar information of a biological/biochemical nature is being

sought.

Use of colloidal gold to tag these reactions may, with care and the addition of

complicated conditions and steps, be conducted in a manner that yields information of a

somewhat qualitative nature. Qualitative uses of these colloidal gold tags however, are

more easily availed of and, in general, yield more accurate and useful results than attempts

to obtain quantitative information with them. To put it another way, colloidal gold in

particular offers outstanding advantages as tag material when the goal is to use it to identify

specific biological molecules or moieties, but obtaining reliable quantitative information from assays and other reactions where only colloidal gold tags are used is most often an

extremely daunting and time-consuming task.

A number of suggestions have been made for employing magnetic beads or particles

as labels for biological reactions, on the premise that magnetic field sensors will yield

readings enabling determinations of, e.g., molecular size or yield of desired end product.

See, e.g. Adelmann U.S. Patent 5,656,429 and Adelmann, L., J. Assn. for Laboratory

Automation 4, No. 3, pp. 32-35 (July 1999).

The Adelmann journal article teaches that off-the-shelf magnetic beads exhibit

"remnant [sic] magnetization, i.e., are permanently magnetizable" (p. 33) and that their

remanent magnetic field is what enables quantitation and/or detection of polynucleotides

and other bound targets. In its examples the beads referred to are of an unusually large size

in the order of 800 nm in one instance and 4 microns in another. Such large beads cannot

effectively be used as markers for the great bulk of biological reactions because their Theological properties prevent their ready movement through the normally somewhat

viscous media in which such reactions occur, and also prevent their ready flow along

matrices such as cellulose derivatives, paper, wood, glass etc. upon which such reactions

are often performed. The article further teaches that scientists in biomedical and

biotechnology laboratories have long recognized a need, when using magnetic beads, for

separating excess unbound beads from those that become bound to target whole cells, DNA

or proteins — and that this separation is performed by subjecting the mixture to a fixed

magnetic field which attracts the unbound beads (Id.) It is noted that both the permanent

magnetizability of these beads and the circumstance that a fixed magnetic field attracts the

excess beads strongly suggest the beads were ferromagnetic in character and not

superparamagnetic .

Baselt U.S. Patent 5,981,297, proposes using magneto-resistive elements, similar to

those used for reading magnetic tapes or disks in, e.g.- electronic and computer

applications, which are described as measuring approximately 20 by 20 μm. (Col. 6 line

34) to detect many particles per element, (as distinguished from a one particle-per-element

embodiment also proposed). This magnetoresistive element is first precoated with an

insulator and a binding molecule specific to the target molecule is then covalently bound thereto. The thus prepared element is placed in a flow cell to which liquid sample

containing target molecule is added, followed by addition of a suspension of 1-5 nm

diameter particles that may be ferromagnetic, ferrimagnetic, superparamagnetic or

paramagnetic and have a coating of the binding molecule specific to the target molecule.

The specification teaches (Col. 7, lines 1-14) that each magnetoresistive element has the purpose to count the particles that bind to the target molecule, and that prior to activating

the magnetoresistive detection element, a magnetic device, such as an electromagnet is used

to remove non-specifically adhering particles. It says this function is "best provided by sending a brief (~10-100 ms) pulse of current generated by a capacitative discharge circuit,

through an air core electromagnet coil" (Id., lines 11-14). A magnetic field generator (Col.

7, lines 21-38) then magnetizes the bound beads, each of which creates a magnetic field that

changes the resistance of the magnetoresistive element to which it is bound and the

resistance is then compared to that of a reference element by a Wheatstone bridge,

whereupon the data are digitized and conveyed to a microprocessor which determines the

total number of beads on the particular magnetoresistive element, from which target

molecule concentration can be calculated.

Various other ways of using magnetic or paramagnetic beads to measure a target substance within a sample have also been described. See, e.g. Rapoport U.S. Patent

5,978,694 involving the use of an electrical conductor to measure changes in magnetic

susceptibility of a liquid sample when the latter, having present a paramagnetic,

diamagnetic or ferromagnetic labelling material (which may be oxygen, which is characterized as "paramagnetic") that is bound at least in part to a target substance, is

subjected to an applied magnetic field.

A German group Kotitz et al. have demonstrated that ferromagnetic (including

ferrimagnetic nanoparticles can be used to tag various biomolecules). See the Kotitz et al.

abstract "Superconducting Quantum Interference Device-Based Magnetic Nanoparticle

Relaxation Measurement as a Novel Tool for the Binding Specific Detection of Biological Binding Reactions" J. Appl. Phys. vol. 81, p. 4317 (April 1977) which relates to a

conference paper given in November 1996 at the 41st Annual Conference on Magnetism and Magnetic Materials, held at Atlanta, Georgia, the related U.S. Patent 6,027,946 which

claims a German priority date of January 27, 1995 and names as inventors the same four

individuals named as authors on the abstract. Also closely related to the published abstract

and the U.S. patent are a further conference paper from the same group published in IEEE

Transactions on Applied Superconductivity vol. 7, no. 2, pt.3, pp. 3678-81 (1997) and first

given at the 1996 Applied Superconductivity Conference in Pittsburgh, Pennsylvania in

August 1996 and a later article from the group published in J. Magnetism and Magnetic

Materials vol. 194, no. 1-3, pp. 62-68 in April 1999. In all of these four cited publications

the group worked with ferromagnetic, including ferrimagnetic, nanoparticles rather than with superparamagnetic particles. Three of these publications including the U.S. Patent,

relate to the detection of analytes labelled with these ferromagnetic materials using a

detection method the authors (inventors) term "magnetorelaxometric" wherein SQUIDS are

used to measure time in milliseconds within which the ferromagnetic particles, after

exposure to, and withdrawal of, a magnetic field — both usually done within a magnetically

shielded environment ~ undergo at least a partial reordering of their magnetic moment.

The IEEE Transactions paper, relates to using SQUIDS to measure remanent sample

magnetization in the absence of an external field. More specifically, this paper describes a

process wherein monoclonal antibodies to collagen Type III were coupled to dextrane - coated ferromagnetic iron oxide nanoparticles having an average mean diameter of 13 nm.

Meanwhile polystyrene tubes were incubated with collagen type III in PBS, whereby this antigenic material adsorbed onto the tube walls.

The ferromagnetic nanoparticle-labelled monoclonal antibodies in a ferrofluid were

added to these prepared tubes and allowed and allowed to incubate for 60 minutes. The

tubes were each exposed to a magnetic field for 10 seconds. A measurement of magnetic remanence was then made on the ferrofluid - filled tubes, the measurement being performed

in a magnetically shielded environment. The tubes were then each decanted and washed

three times with PBS and a magnetic remanence measurement was again made on each.

The results showed no change in the remanence signals from those obtained while the tubes

were still filled with ferrofluid. From this it was concluded that unbound particles, — i.e.

particles that did not participate in the antibody-antigen reaction ~ could be wholly

disregarded whether or not they had initially reacted with monoclonal antibody alone to

form "blocked particles" and that the measured remanent effect was solely discernible with

particle labelled antibodies that bound to the adsorbed antigen on the cell walls. This

measured remanent magnetization, moreover, was found to be a linear function of antigen

concentration the tube walls.

The present invention involves the discovery that individual superparamagnetic

particles of physical size such that they exhibit average mean diameters between 1 and

about 100 nm, preferably between 1 and about 60nm, and most preferably from 5 nm to 50

nm are not permanently magnetizable (and hence do not possess the remanent magnetization

described in the IEEE publication) but nevertheless do acquire, when closely packed

together in an interlocked bio-organic matrix, an impermanent magnetization effect that

persists long enough to permit informative and highly useful measurements to be made. BRIEF DESCRIPTION OF THE INVENTION

The present invention involves using superparamagnetic particles having a physical

size as measured by X-ray diffraction and transmission electron microscopy wherein the

average mean diameter is in the range from 1 to about 100 nm, preferably 1 to about 60 nm

and most preferably between about 5 nm and 50 nm, as labeling agents for at least one

selected or suspected reactant of a biological reaction wherein the reactants, upon

interaction among or between them, form a three-dimensional mass of tightly packed, often

molecularly cross-linked, bioorganic material and bound superparamagnetic (SPM)

particles. In such a mass, the SPM particles are closely juxtaposed to one another in all

three dimensions of the mass and the result is that, upon subjecting the mass to the

magnetizing effect of a magnetic field (e.g., the field exercised by a magnet of about

10,000 Gauss in strength) for a short period in the order of not more than 30 seconds, preferably 10 seconds or less, the closely juxtaposed particles become magnetized.

It has been experimentally shown that this magnetization gradually decays over a

period of at least 20-30 minutes and in some cases, longer, to a point where it disappears.

The magnetization described in the preceding sentence is referred to herein as

"nonpermanent aggregative magnetization. " This non-permanent aggregative magnetization is desirably measured within a set time interval, in minutes, of the withdrawal of the

influence of the magnetic field from the mass.

By standardizing this interval to 5 minutes during the work underlying this

invention, it was found that a standard curve could be constructed for particles having the

same identity characteristics (i.e., particle diameter, surface treatment, and identity of biomolecule to which it is bound) that permits ready calculation of the number or

concentration of target molecules that reacted with these particles and were accordingly

present in the test sample.

In these experiments it was also found that stray individual particles of

superparamagnetic material having a physical size with the same average mean diameter

range and having the same surface treatment, whether initially bound to a biomolecule

identical to those that actually reacted with the target molecules of the test sample or wholly

unbound, did not retain measurable magnetization upon withdrawal of the magnetic field.

Because they did not, it was concluded to be unnecessary to perform any step of physically

separating them prior to proceeding with measurement of the nonpermanent aggregative

magnetization of the agglomerated interacted mass of superparamagnetic particles and bioorganic molecules. The latter produce measurable magnetic readings, it is theorized,

because of their lower sensitivity to thermal effects. The unreacted superparamagnetic

labelled antibody or other biomolecule has a smaller physical particle size compared to the

superparamagnetic-labelled immunocomplex or other superparamagnetic-labelled reacted

biomass. The superparamagnetism of the labelled antibody or other biomolecule and the direction of its magnetization are more susceptible to immediate dissipation as a result of

thermal effects and, just as in the superparamagnetic particle alone, it is believed that the

direction of magnetization in the superparamagnetic-labelled antibody tends to become

random very quickly. BRIEF DESCRIPTION OF THE DRAWINGS

The attached drawings are as follows:

Figure 1 is a schematic diagram of a hysteresis curve for typical ferromagnetic

material taken from the literature. In the diagram, H represents the applied magnetic field

in oersteds and σ represents the magnetization resulting therefrom. Magnetic saturation is

represented by σ_s while the remanence, or magnetic induction remaining after removal of

the magnetic field is represented by σ_r. The reverse magnetic field necessary to bring σ_r to

zero is represented by H_c, the coercive force.

Figure 2 is a superimposed plot of the measured hysteresis of superparamagnetic

particles coated with small polyacrylic acid and then reacted with a commercially available

antibody called bethyl. Three such measurements were made on identically treated

particles of three different iron concentrations as measured on particles in suspension — i.e.

2.4 mg/ml. (circles on the plot), 1.0 mg/ml. (squares on the plot) and 0.3 mg/ml. (triangles

on the plot). In the plot M represents the applied magnetic field in emu (electromotive

units) per cc. ("cm³") while H is the coercive force in oersteds necessary to impose

magnetization (rightward direction from the zero line) or reverse magnetization (leftward

from the zero line).

Figure 2A is a similar plot of superimposed hysteresis measurements made using

two different instrumental measuring techniques on polymer coated - ( e. , small

polyacrylic acid- coated or chondroitin sulfate A-coated) superparamagnetic Fe₃ O₄ particles which particles before coating were identical to those used in the experimental work

described herein and a hysteresis measurement made with a Vibrating Sample Magnetometer ("VSM") on superparamagnetic particles of Fe₃ O₄ conjugated to CPS

antibody. In Figure 2A, the polymer-coated superparamagnetic Fe₃ O₄ particle

measurements made using a SQUID instrument are represented by solid black squares; the

measurements made on polymer-coated superparamagnetic Fe₃ O₄ particles are represented

by white circles and the measurements of superparamagnetic Fe₃ O₄ particle-antibody

conjugate are represented by solid black triangles.

Figure 3 is a plot of measured magnetization in relative magnetic units vs. antigen

concentration in ng/ml of two series of superparamagnetic particle: antigen: immobilized

antibody sandwiches as measured at the capture line of an ICT device of the type described, e.g., in U.S. Application Serial No. 07/706,639 of Howard Chandler, now U. S. Patent

, or any of its various continuing patents and applications, all of

which are assigned to Smith Kline Diagnostics, Inc. but exclusively licensed to the assignee

of this invention, Binax, Inc. for immunological assays over a wide subject matter area.

These data were obtained from work described in Example 3.

Figure 4 is a plot of Fe concentration, calculated as Fe₃ O₄, of the

superparamagnetic particle labels used in Example 3 in g/ml against measured CHW

antigen concentration in ng/ml. This data was also obtained from work described in

Example 3.

Figure 5 is a plot of measured magnetization in relative magnetic units against

antigen concentration in pg/ml. It also embodies data from work described in Example 3.

Figure 6 is an X-ray diffraction diagram showing superimposed results of measuring

particle size of the superparamagnetic Fe₃ O₄ particles used in the experimental mental work herein described (pattern A) the same particles after coating with one of the two polymers

identified above (pattern B).

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, superparamagnetic particles which are

individually too small to maintain any degree of magnetization after exposure to the action

of a magnetic field of the strength of about 10,000 Gauss for a period as short as possible,

preferably 10 seconds or less, and not more than 30 seconds, have been shown to acquire

measurable non-permanent aggregative magnetization — i.e., collective magnetization of an

aggregated, interacted three dimensional biomass — when closely incorporated into a tightly .

packed three-dimensional mass with agglomerated biological material such as, e.g., a mass

of labeled superparamagnetic antibody: antigen: immobilized antibody "sandwiches", a

clotted mass of labelled blood platelets, a mass of chromatographically separated protein, and the like.

Use of superparamagnetic particles as labels for biological, including biochemical,

reactions offers substantial advantages over many of the labels now used, e.g., in various

assay systems. For example, superparamagnetic particles, in contrast to ferromagnetic

particles, do not display remanent magnetization and have no magnetic properties until subjected to the influence of a magnetic field. They are accordingly virtually unlimitedly

shelf-stable in contrast to many of the labelling materials in common use, including

colloidal metals, enzymes, chemiluminescent agents, radioactive tracers etc.

Their stability renders them easy to mix with other substances, to suspend freely in

liquids and otherwise to work with, so long as they are not exposed to magnetic fields of sufficient intensity to excite magnetization.

Nonpermanent aggregative magnetization as observed in the context of the present

invention is a measurable phenomenon which is a straight line function of the

concentration, or number, of target biomolecules in a test sample. However, care must be

taken to measure the nonpermanent aggregative magnetization at the same time interval

after removal of the magnetic field that causes this magnetization if one is to achieve

comparable results in a series of tests - e.g., tests conducted at different concentrations of a target analyte molecule, tests undertaken to construct a standard curve, tests undertaken

with the intent to rely on an already constructed standard curve to determine concentration

present in a sample, etc.

It is believed that particle size, surface features of the particles, magnetic field strength and time employed in the magnetization step, as well as the mean distance between

the magnetized particles trapped in the end product mass of bioorganic material and bound

particles, will all play a role in the length of time within which nonpermanent aggregative

magnetization persists and the rate at which it decays. It also appears that the decay, at

least in systems so far tested, occurs at a rate such that correlation of the number of bound

particles with the concentration of a target analyte or other target molecule can be achieved

when total magnetization measurements are made not only at the 5 minute interval

following the magnetization step that was chosen for the work underlying this invention but

at some other uniform interval from that step. Care must be taken, of course, that the

measurements are taken at an interval such that measurement of total magnetization yields

readings that are in excess of the reading for any background magnetization that may need to be deducted, depending upon the "platform" or biological matrix that may be present. Furthermore, before selecting a different interval for measurement of total magnetization,

one needs to ensure that the rate of decay of non-permanent aggregative magnetization

follows a consistent pattern for superparamagnetic particles that have the same treatment

history.

As used herein, "superparamagnetic particles" refers to particles that are magnetizable but retain no permanent magnetization when tightly packed together in close

association in a mass of inter-reacted bio-organic materials and that, when measured

individually after attempted magnetization, exhibit no remanent magnetization. These

particles may comprise pure metals such as, Fe, Co, Cd, Ru, Mg, Mn, etc. that are known

to be readily magnetizable, iron oxide, CoFe₂ O₄, MgFe₂ O₄ and oxides of other metals that

are known readily to be magnetizable when a mass thereof is subjected to the influence of a

magnetic field, such as nickel, cadmium, cobalt, ruthenium, etc. Also usable are, e.g. Fe-

Ru and its oxides and other metal combinations, and oxides thereof, that exhibit spinel

structure upon examination by X-ray diffraction and Transmission Electron Microscopy. It

should be noted that pure metals are superparamagnetic only within a physical size range

wherein the average mean diameters are confined with a few nanometers, usually less than

5 nm. Superparamagnetic particles of pure metals are also chemically unstable whereas

oxides of corresponding superparamagnetic particles are relatively inert and maintain their superparamagnetism within a broader physical size range.

These superparamagnetic particles are not intrinsically reactive with bio-organic

materials and often are desirably coated with a substance that enables them to react with a binding partner of the target molecule which is to be monitored, assayed for, or otherwise located and quantitated. Various methods of and materials for such coating are known and

have been used in the past for coating polymers or glass, including glass beads and solid

polymer - comprising inserts or "dipsticks" that have been used in various assay systems.

The same coating methods and materials are useful in coating superparamagnetic particles

to be used in detecting end products of biological, including biochemical, reactions.

Various methods of adsorption are also well known wherein proteins and the like are

directly adsorbed on, e.g. iron oxides and the like and they also may be utilized in this

invention to improve the reactivity of the particles.

Superparamagnetic particles are distinguished from both ferromagnetic, including

(ferrimagnetic), particles, which acquire permanent remanent magnetization upon exposure

to an external magnetic field, and also from paramagnetic materials, which have a positive

magnetic susceptibility less than 0.001 times that of ferromagnetic materials. The magnetic

susceptibility of superparamagnetic materials lies between that of ferromagnetic materials

and particles is intermediate that of ferromagnetic and paramagnetic particles. Ideal

superparamagnetic systems, at temperatures equal to or below their critical blocking

temperature exhibit a slow relaxation time — i.e., they revert from a magnetized state to a

non-magnetized state slowly. The particles used in the work underlying this application

were of 5-15 nm average mean diameter as measured by X-ray diffraction and

Transmission Electron Microscopy and exhibited a blocking temperature slightly above

room temperature, i.e., slightly above about 25° C. They were of pure Fe₃ O₄ having spinel

structure, as confirmed by X-ray diffraction and transmitted electron microscopy. Superparamagnetic materials are known by physicists not to exhibit remanent

magnetization. The hysteresis loop of superparamagnetic materials (i.e., the plot obtained

by plotting magnetization against magnetic field strength) is curve-like and it typically

resembles those shown in Figures 2 and 2A hereof. This is in contrast to the typical

hysteresis loop of ferromagnetic materials (Figure 1) and the linear hysteresis plot obtained

with paramagnetic materials. Most usually, superparamagnetic particles have a small

average mean diameter in the order of less than 50 nm, often 30 nm or less, in physical size

as measured by X-ray diffraction and Transmission Electron Microscopy — although in

some systems larger sizes of particles with superparamagnetic properties have been

observed. (It is noted parenthetically that both their size and behavior after removal from a

magnetic field suggest that the 800 nm and larger particles referred to in the Adelmann et

al. article were ferromagnetic and not superparamagnetic). Still further, it is typical of

superparamagnetic particles that the magnetization they may exhibit when subjected to a

magnetic field, decays with time until it dissipates altogether. Finally, superparamagnetic

particles possess a degree of magnetic ordering - i.e., they have what is called a subdomain

structure consisting of clusters of varying sizes containing some atoms with unpaired

electrons in the unmagnetized state, but the clusters are small and scattered in comparison to the "domain structure" of ferromagnetic materials which are characterized by larger

clusters called domains in which each atom or other structural unit has unpaired electrons

that impart a net magnetic moment. In these latter materials, each domain exhibits a

directional magnetic effect which is the vector sum of all unpaired electrons present in that

domain. In sum, ferromagnetic materials have strong magnetic ordering, superparamagnetic

materials have some magnetic ordering, but much less than ferromagnetic materials. The

presence of magnetic ordering in superparamagnetic materials has been confirmed by

neutron diffraction measurements. See Chen et al. , "Synthesis of Paramagnetic MgFe₂ O₄

Nanoparticles by Coprecipitation", J. Magnetism and Magnetic Materials, vol. 94, pp. 1-7

(1999). Paramagnetic materials have no magnetic ordering.

For a good technical description of similarities and differences physicists recognize among "ferromagnetic", "superparamagnetic", and "paramagnetic" materials, see Chen et

al., "Size-dependent Superparamagnetic Properties of Mg₂Fe O₄ Spinel Ferrite

Nanocrystallites", Appl. Phys. Letters, vol. 73, pp. 3156-8 (1998).

It is possible that the ability to measure non-permanent aggregative magnetization in

agglomerated three-dimensional reaction products comprising bio-organic materials in close association with superparamagnetic particles is attributable to the relatively stable

macrostructure of these reaction products, which macrostructure holds the incorporated

particles in place and prolongs the decay, of the magnetization imparted by

exposure to a magnetic field. Applicants, however, have not established this or any other

scientific explanation for the reproducible phenomenon observed in the experimental work

relating to this invention and hence do not intend to bind themselves to any particular

explanation.

The present invention provides enhancements in a virtually unlimited spectrum of in

vitro biological reactions including at least immunoassays, DNA probes, oligonucleotide probes, chromatographic molecular separations and other biological reactions where it is desirable to quantitate the amount of a target molecule present. It is believed that this

invention may also be useful in monitoring certain in vivo biological reactions. The

detection system of this invention is beneficially employed in immunoassays generally, but

especially in immunochromatographic and other "lateral flow" assays and in so-called "flow

through" assays — i.e., those involving vertical flow steps in which the reactants are

brought together.

The Midwest Scientific Co. newsletter, Shark Bytes, for October 2000 describes a

form of assay now in development at Ohio State University wherein a compact disc ("CD")

rotated by a compact disc player is equipped with tiny reservoirs and channels that cause

medical samples suspected of containing target analyte to mix with tiny pools of test

reagents. Including superparamagnetic particle tagged binding partners for analytes

suspected of being present in tiny pools on such test platforms would lead to very useful

assays capable of being rapidly performed and rapidly evaluated via the computer anticipated to be included in the CD player of the Ohio State system. This computer could

readily be programmed to read non-permanent aggregative magnetization imparted by an

external magnetic field in digital form and to correlate this reading to stored information

corresponding to a standard curve. As those skilled in immunoassays will recognize, one

CD with associated CD player-computer combination could readily be adapted to perform

several assays simultaneously on portions of a single test sample by providing, e.g.,

different antibodies for different target analytes conjugated to superparamagnetic particles

placed in different "pool" regions of the CD.

In addition to CD's, other "platform" materials upon which superparamagnetic particle-tagged biomolecules may be reacted with target biomolecules in a test sample are

contemplated to be useful in work performed within the scope of this invention.

Possibilities specifically explored in preliminary work, in addition to what the specific

examples below show, are balsa wood and glass. With balsa wood, it was found that even

though the material is intrinsically non-magnetic and non-magnetizable its capillarity may

lead to readings of non-permanent aggregative magnetization that exhibit a very large

standard deviation. It is believed that filling these capillaries with non-magnetizable plastic

or with a substance such as bovine serum albumin, other proteins, poly ethylenegly col or

other substances well known for blocking capillarity in "dipstick" type immunoassay

devices described in the prior art would render balsa wood more acceptable as a platform.

Glass was found to avoid the capillarity problem and to be a satisfactory platform material,

provided that appropriate background readings are obtained, allowing one to compensate

for the fact that most glass slides contain sufficient iron to be magnetizable to a low degree

upon exposure to a magnetic field. This makes it necessary to determine the background

signal and subtract it from sample readings whenever biological reactions wherein one

reactant is labelled with superparamagnetic particles according to this invention are run on

glass as a platform.

To measure non-permanent aggregative magnetization, various instruments may be

used. In this regard, several different research and development groups are in process of

developing relatively low cost measuring instruments which apply knowledge gleaned from

high resolution magnetic recording technology and computer disk drive technology. One of

these is the Ericomp Maglab 2000, at least one early prototype version of which is illustrated in the Adelman J. Assn. for Lab. Automation article cited hereinabove. Another

is the Quantum Design, Inc. instrument described in U.S. Patent 6,046,585 issued April 4, 2000.

The following specific examples illustrate the substitution of superparamagnetic

labelling according to this invention for colloidal gold in an immunochromatographic

("ICT") assay for Canine Heart Worm that is commercially available from Binax, Inc,

assignee of this patent application.

Example 1 - Selection of Coating Agent for Superparamagnetic Particles

Superparamagnetic particles can be coated with a reagent that has free carboxyl

functional groups in order to be capable of covalent coupling to a particular antibody, such

as

the antibody Canine Heart Worm ("CHW"). Initial work was accordingly performed to ascertain the coating material of choice for this purpose.

Because rheological properties are important to the successfiil operation of ICT

assays and past experience with colloidal gold labels has shown smaller particles to be

Theologically

superior, 10 nm diameter Fe₃ O₄ particles were chosen for this work. Their size was

confirmed by X-ray diffraction and by Transmission Electron Microscopy.

Two polymeric coating materials were tested on separate lots of superparamagnetic

particles using the coating method described in U.S. Patent 5,547,682. One lot of these

particles was coated with Chondroitin Sulfate A ("CSA") and the other with small polyacrylic acid ("sPAA"). The coated particles in suspension, in each instance had a diameter of 40 to 60 nm, as determined by dynamic light scattering. Both the CSA - coated

and the sPAA coated superparamagnetic particles were further tested and it was shown

thereby that CSA - coated particles were superior in stability and ability to bind to

antibodies. The hysteresis loop as measured by Vibrating Sample Magnetometer ("VSM")

tests for three sets of particles having varying iron concentrations, all of which were sPAA

coated and had the commercially available antibody bethyl bonded thereto is shown in

Figure 2. These particles in uncoated form are identical to those which were the starting

materials for Example 3. Figure 2 shows the typical hysteresis curve shape of

superparamagnetic materials and also confirms that these particles had no remanent

magnetization. Figure 2A shows hysteresis measurements made with coated particles of

superparamagnetic lOnm Fe₃ O₄ with a SQUID instrument (curve with black squares) and

by VSM (curve with white circles). Also, shown on Figure 2A is a plot of hysteresis

measurements made by VSM of the same coated particles to which a carboxy

polysaccharide antibody was conjugated. All three again exhibit the typical shape of

superparamagnetic material hysteresis behavior and confirm the particles lack of remanent

magnetization.

Example 2 - Preparation of Superparamagnetic Particle - Labelled Antibodies

CSA - coated superparamagnetic particles prepared as in Example 1 were covalently

coupled to anti-Canine Heart Worm ("CHW") antibody identical to the anti-CHW antibody

used in the manufacture of the commercially available Binax ICT test for CHW antigen and

were then suspended in phosphate-buffered saline solution containing 10 mg/ml of bovine

serum albumin pending their use as in Example 3. Example 3 - Performance of ICT Assay for CHW Using Superparamagnetic Labels

ICT flow path test strips of nitrocellulose were treated in the same manner as those

used in the Binax commercially available ICT assay for CHW. These strips were

incorporated into dipstick-type devices by the lamination of absorbent pad components overlapping opposite ends of the flow path test strip. A series of solutions containing CHW

antigen were prepared with antigen concentrations ranging from 100 pg/ml to 200 ng/ml.

190μl of each of these antigen solutions were dispensed into separate wells of a new

polystyrene 96 well microtiter plate. To these samples were added 5μl of the CS A-coated

superparamagnetic particle-labelled anti-CHW antibodies of example 2. Labeled

antibody /antigen solution mixtures were incubated for 15 minutes at room temperature.

The sample receiving end of a dipstick device was added to each labeled antibody/antigen

solution mixture immediately following this incubation, causing the mixture to flow into the

capture zone. Immobilized unlabelled rabbit polyclonal anti-CHW antibodies bound to the

strip in the capture zone thereupon reacted with antigen: superparamagnetic labelled

antibody conjugates to form immobilized antibody: antigen: superparamagnetic labelled

antibody "sandwiches" along the capture line. The ICT strips were removed from the ICT

devices after 15 minutes, and exposed to a magnetic field of 10,000 Gauss for 10 seconds

each. After 5 minutes from the removal of the magnetic field, each strip was placed in an

Ericomp Maglab 2000 instrument with the capture line in the field of view of the detector

and its non-permanent aggregative magnetization was read. Each antigen solution was tested in duplicate in the ICT test as described. The

readings of non-permanent aggregative magnetization for both series of sample having

known antigen concentrations above 1 ng/ml have been graphed in Figure 3 against antigen concentration. The Fe content of the immune complexes at the capture lines of ICT

devices used in the duplicate series of tests was determined by a chemical calorimetric

procedure using a commercial ferrizine test from Sigma Chemical Co. It was found that

the calorimetric chemical test results and the magnetization readings correlated well, as shown in Figure 4, a plot of iron concentration calculated as Fe₃ O₄, in g/ml, against

antigen concentration in ng/ml.

In the ICT tests as performed in this example, any labelled antibody initially

deposited at the flow path threshold that fails to react with antigen in the sample flows past

the capture zone and into another pad positioned upstream from that zone. These unreacted paramagnetic particle-labelled antibodies were subjected to the effect of a magnetic field of

10,000 Gauss for 10 seconds, set aside for 5 minutes and then placed in the sensor area of

the Ericomp Maglab

2000 instrument and found to exhibit no measurable magnetization.

From the results of the foregoing examples, it was determined that the relationship

between magnetic reading in relative magnetic units ("RMU") and concentration of antigen

(the target analyte) is an essentially linear function in the range of 1 ng antigen/ml to 150

ng antigen/ml. See Figure 3, a plot of relative magnetic units against antigen concentration

in ng per ml for each of the two series of CHW assays performed. The instrument noise,

however, caused large standard deviations in the readings of samples having concentrations of antigen below 1 ng/ml. This is illustrated in Figure 5, a plot of measured values in

relative magnetic units against antigen concentration in pg per ml. The fact that ICT strips

having 200 pg/ml of added antigen had non-permanent aggregative magnetization that could

be read when that antigen was incorporated in labelled antibody: antigen: immobilized

antibody sandwiches collected in a mass, subjected to 10,000 Gauss of magnetic field for

10 seconds, and then set aside for five minutes is an indication nonetheless that the

sensitivity of the test is significantly enhanced by substituting superparamagnetic labels for

colloidal gold labels.

With an improved instrument having reduced noise, it is clear that the physical

sensitivity of superparamagnetic labelling as described approaches 0.1 ng of Fe calculated

as Fe₃ O₄ or about 10^"18per mole, while the broad dynamic sensitivity range will fall

between about 1 and 10⁶ relative units and has potentially high tolerance to interference

from various biological matrices that may present.

While the invention has been exemplified in the context of a well known

immunodiagnostic system specific to the antigen of the causative agent for the canine

disease Dirofilaria immitis, the vast range of applications in which it will produce greatly

improved results or will enable precise quantitative measurement of observed phenomena

previously deemed to be difficult to impossible to measure will be readily apparent to those

ordinarily skilled in hnmunochemistry and/or biology. It is accordingly intended that the

scope of this invention be limited only to the extent of the scope of the appended claims.

Claims

WE CLAIM:

1. A process for detecting a biological reaction which comprises:

(a) conjugating or adsorbing to each of a group of superparamagnetic particles

identical biomolecules which are members of a biological binding pair,

(b) contacting the product of step (a) with a sample selected from among liquids

and solids, containing or suspected of containing molecules which comprise the biological

binding partner of the biomolecules conjugated to or adsorbed on the superparamagnetic

particles,

(c) permitting the superparamagnetic particle: biomolecule conjugates or

superparamagnetic particle: biomolecule adsorbates from step (a) to react with any

biological binding partner molecules present in the aforementioned sample to form a

complex, tightly bound, three-dimensional mass comprising interlinked biomolecules and

bound superparamagnetic particles;

(d) exposing the said mass to a magnetic field for the shortest period necessary to induce magnetization of the superparamagnetic particles in said mass and then

immediately removing the magnetic field, whereupon the superparamagnetic particles in

said mass exhibit in concert measurable nonpermanent aggregative magnetization which

persists for a period of at least 20 minutes following exposure to the magnetic field, and

either

(e) confirming the presence of such magnetization with a suitable instrument if

only a qualitative result is desired, or (f) measuring the intensity of the magnetic signal of the said nonpermanent

aggregative magnetization before it dissipates and correlating it to the quantitative

concentration, or number, of one of the biomolecules of step (a) or step (b) that participated

in forming the mass referred to in step (c).

2. A process according to Claim 1 in which the superparamagnetic particles comprise Fe₃ O₄ particles having an average mean diameter as measured by X-ray diffraction and

Transmission Electron Microscopy of 1 nm to about 100 nm.

3. A process according to Claim 2 in which each superparamagnetic particle is

conjugated to an antibody, the sample in step (b) is a liquid sample which contains the

antigen that is the specific binding partner of said antibody, and a quantitative result is

obtained by performing step (f).

4. A process according to Claim 3 in which the period of exposure to a magnetic field

in step (d) is 5-10 seconds and the average mean diameter of the superparamagnetic

particles as determined by X-ray diffraction and Transmission Electron Microscopy is in the range from 5 nm to 60 nm.

5. A process according to Claim (4) which is an -nmunoassay in which the antigen

content of the sample is quantified in step (f).

6. A process according to Claim 5 which is conducted in lateral flow format.

7. A process according to Claim 6 which is conducted in the format of an

immunochromatographic assay.

8. A process according to Claim 5 which is conducted in a vertical flow or flow- through format.

9. A process according to Claim 1 in which the superparamagnetic particles comprise

those selected from among superparamagnetic particles of a single magnetizable metal or

superparamagnetic particles of two combined magnetizable metals or superparamagnetic

particles of oxides of either a single magnetizable metal or two combined magnetizable

metals.

10. A process according to Claim 9 in which each superparamagnetic particle is

obtained by performing step (f).

11. A process according to Claim 10 in which the period of exposure to a magnetic field

particles as determined by X-ray diffraction and Transmission Electron Microscopy is in

the range from 5 nm to 50 nm.

12. A process according to Claim 11 which is an immunoassay in which the antigen

content of the sample is quantified in step (f).

13. A process according to Claim 12 which is conducted in lateral flow format.

14. A process according to Claim 13 which is conducted in the format of an

immunochromatographic assay.

15. A process according to Claim 12 which is conducted in a vertical flow or flow

through format.

16. A process according to Claim 9 in which the superparamagnetic particles comprise

superparamagnetic particles of an oxide two combined magnetizable metals, which particles

exhibit a spinel structure as determined by X-ray diffraction analysis and Transmission

Electron Microscopy.

17. A process according to Claim 16 in which each superparamagnetic particle is

obtained by performing step (f).

18. A process according to Claim 17 in which the period of exposure to a magnetic field

the range from 5 nm to 50 nm.

19. A process according to Claim 18 which is an immunoassay in which the antigen

content of the sample is quantified in step (f).

20. A process according to Claim 19 which is conducted in lateral flow format.

21. A process according to Claim 20 which is conducted in the format of an

immunochromatographic assay.

22. A process according to Claim 19 which is conducted in a vertical flow or flow

through format.

23. A process according to Claim 2 in which identical biomolecules are adsorbed to superparamagnetic particles in step (a).

24. A process according to Claim 23 in which the period of exposure to a magnetic field in step (d) is 5-10 seconds and the average mean diameter of the superparamagnetic

the range of 5 nm to 50 nm.

25. A process according to Claim 24 which is an immunoassay.

26. A process according to Claim 25 which is conducted in lateral flow format.

27. A process according to Claim 26 which is conducted in the format of an immunochromatographic assay.

28. A process according to Claim 26 which is conducted in a vertical flow o flow-

through format.

29. A process according to Claim 23 in which the superparamagnetic particles comprise those selected from among superparamagnetic particles or superparamagnetic particles of

two combined magnetizable metals or superparamagnetic particles of oxides of either a

single magnetizable metal or two combine magnetizable metals.

30. A process according to Claim 23 in which the superparamagnetic particles comprise

superparamagnetic particles of an oxide of two combined magnetizable metals, which

particles exhibit a spinel structure as determined by X-ray diffraction analysis and

Transmission Electron Microscopy.

31. A process according to Claim 1 wherein step f is performed instead of step (e).

32. A process according to Claim 31 which is repeatedly performed on a series of

samples each containing different concentrations of a given biological binding partner, as

referred to in step (b) of Claim 1, wherein the measurement in step (f) of the intensity of

the magnetic signal from the nonpermanent aggregative magnetization of the mass referred to in step (d) of Claim 1 is performed uniformly for each sample at the same time interval from the time of removal from the magnetic field of exposure of the mass referred to in

each of steps (c) and (d) of Claim 1.

33. A process according to Claim 31 wherein, once a correlation has been established

between the concentration, or number, of the molecules of given biological binding partner,

as referred to in step (b) of Claim 1, and the intensity of the magnetic signal of the

nonpermanent aggregative magnetization of the mass containing it referred to in steps (c)

and (d) of Claim 1 by measuring said signal in step (f) at a uniform time interval for a

series of samples has been obtained as recited in Claim 32, the same uniform time interval

in step (f) is adhered to whenever the process of Claim 31 is performed upon any sample

containing an unknown concentration, or number, of molecules of the same biological

binding partner.

34. A process according to Claim 1 wherein the average mean diameter of the

superparamagnetic particles as determined by X-ray diffraction and Transmission Electron

Microscopy is in the range of 1 nm to 60 nm.